Otimização do preparo e caracterização de monolitos de silica aninhados em Esponja Mágica para uso como sorvente extrator

Abstract

Monolithic sorbents have enabled greater versatility in sample preparation for chemical analysis. Monoliths can be prepared inside molds with high reproducibility in various sizes and shapes, and their macroporosity allows the flow of liquids through them, resulting in a high interaction capacity and high concentration factor of the analytes. Silica-based monoliths have high mechanical strength and are easy to modify, allowing fine tuning of sorbent selectivity. However, silica monoliths become brittle and shrink significantly when they have dimensions larger than those produced inside capillary tubes. Thus, the polycondensation of organosilanes inside sponges is a simple and efficient strategy for preparing these monoliths for sample preparation in the desired format. In this work, a central composite design was performed to optimize the synthesis of silica monoliths inside melamine-formaldehyde sponges for use in sorptive extractions with silica monolith nested in sponge. The melamine sponges were treated with 1% HNO3 solution to prepare the silica monoliths. Then, the precursor mixture with tetraethoxysilane (TEOS) and urea was added into 1 cm3 cubes of the pretreated sponges. The sponges soaked with the monolithic solution were subjected to heat treatment under different conditions according to the experimental design. The best preparation conditions that resulted in the highest mass, approximately 70 mg cm-3 of silica monolith in the sponges, were a condensation temperature of 60 °C, a urea mass of 40 mg, and 500 μL of TEOS. The silica monoliths nested in the sponge under optimized conditions were evaluated by optical stereomicroscopy, infrared spectroscopy, thermogravimetric analysis, macroporosity, specific surface area and scanning electron microscopy, and it was possible to observe the formation of the silica monolithic structure in the sponge pores (87.5% occupation of 1 cm3 of sponge), with characteristic signs of silanol groups and siloxane bonds. The silica monoliths presented a needle shape, with a surface area of approximately 160 m2/g and a structure containing micro- and macropores. In general, it is observed that the silica monoliths nested in the melamine sponge present a rigid structure, making their application in extraction processes viable, which would not be possible in the form of autonomous monoliths (without nesting).